1. Field of the Invention
The present invention relates generally to a device and method for outer-loop and closed-loop power control of a CDMA (Code Division Multiple Access) mobile communication system in DTX (Discontinuous Transmission) mode, and in particular, to a device and method for continuous outer-loop power control of a physical channel which transmits only power control bits (PCBs) without data.
2. Description of the Related Art
In the IS-2000 standard, packet transmission is implemented in three ways: the P1 option using a fundamental traffic channel and a supplemental channel (SCH), the P2 option using a fundamental traffic channel and a dedicated control channel (DCCH), and the P3 option using a fundamental traffic channel, a DCCH, and an SCH. A traffic channel and a DCCH transmit control information about packets and a signaling message and an SCH transmits packet data. The control information and the signaling message are generated discontinuously. In the absence of data traffic, the fundamental traffic channel transmits null traffic. A forward DCCH transmits only power control bits and a reverse DCCH transmits only pilot symbols and power control bits if there is no data traffic to be transmitted. That is, the DCCH only transmits data when there is data to transmit. This is called a DTX mode.
Meanwhile, in comparison with the W-CDMA (Wideband-CDMA standard according to the 3GPP, the Third Generation Partnership Project), the Japanese and European IMT-2000 (International Mobile Telecommunications 2000) standard, the conventional IS-95 CDMA mobile communication system that mainly provides voice service releases a channel once the data is completely transmitted and requests a channel and connects to the channel when necessary. However, the conventional method cannot provide other high quality services like packet data service because it involves many delay factors such as reconnection delay time. Therefore, other services including packet data service should be provided in a new method. In many cases, data is intermittently transmitted during packet data service such as Internet access and file download. As a result, a non-transmission period is interposed between packet data transmission periods. According to the conventional technology, a dedicated data channel is released or maintained for the non-transmission period. In the former case, a long time is taken to reconnect a channel, making it impossible to provide a corresponding service; whereas, in the latter case, channels which are transmitting no data, are maintained, drastically reducing usage efficiency of the system. This problem can be solved by establishing a DCCH between a base station and a mobile station to transmit and receive a control signal associated with a dedicated data channel for a transmission period, and to be maintained when the dedicated data channel released for a non-transmission period, so that channels can be used efficiently and channel reconnection occurs rapidly upon generation of transmission data. This state is termed the control only substate.
An outer power control loop and a closed power control loop are used together for power control in a DTX mode. The closed power control loop controls power in power control group (PCG) units using a threshold fixed for each frame, while the outer power control loop varies a threshold depending on the presence or absence of frame errors and feeds the threshold to a closed-loop power controller. In the case of using both the outer power control loop and the closed power control loop while in DTX mode, a closed-loop power control is executed using a threshold determined in the outer power control loop upon generated of frames, and using the previous threshold in the absence of frames to transmit.
There will be given a description of the outer-loop power control and the closed-loop power control in a DTX mode.
FIGS. 1A and 1B are block diagrams of a transmitter in a typical CDMA mobile communication system. Insertion of PCBs in the DTX mode will be described referring to FIGS. 1A and 1B.
A control message buffer 111 is a memory for temporarily storing a control message transmitted for communication of a control message on a DCCH. The message buffer 111 is configured to have a capacity enough to store one or more frames. The control message buffer 111 interfaces a control message between a higher layer processor and a MODEM controller 113. The higher layer processor stores a control message added with header information to identify a frame according to a message type in the control message buffer 111, sets a flag to notify the storage, reads the control message from the control message buffer 113, and clears the flag to notify the read, thereby preventing over-write and over-read.
The MODEM controller 113 reads the control message from the control message buffer 111, the MODEM controller 113 determines a message type by analyzing the header of the control message, and outputs a payload to be transmitted on a DCCH according to the message type and a corresponding control signal. The control message is variable in duration according to the analysis result, usually 5 or 20 ms. The MODEM controller 113 determines whether there is a control message to transmit and controls transmission of the DCCH. That is, the MODEM controller 113 generates a first gain control value for signal Gc in the presence of a control message to be transmitted and a second gain control value for signal Gc for blocking signal transmission on the DCCH in the absence of a control message. The gain control signal Gc is for controlling the transmission power of the DCCH. While the gain controller is located at the front end of a spreader, the same effect can be produced even if it is at the rear end of the spreader.
The MODEM controller 113 controls transmission of the DCCH while in DTX mode. That is, the MODEM controller 113 performs a DTX mode control according to the capacities of the data service signals and MAC (medium access control) related messages communicated on the DCCH, to thereby use channel capacity efficiently. Since voice traffic and signal traffic are multiplexed in IS-95, both a voice channel and a signaling channel should be opened all the time for data service. However, the DCCH operates in the DTX mode and thus need not be opened for a control signal If no signaling information to be transmitted exists, a DTX gain controller reduces transmission power for efficient use of radio resources.
A CRC (Cyclic Redundancy Check) generator 115 adds a CRC to the control message received from the MODEM controller 113 to allow a receiver to determine the quality of a frame, that is, the presence or absence of a frame error. The CRC generator 115 outputs a control message with the CRC under the control of the MODEM controller 113. A 40-bit control message with a 16-bit CRC is generated for a 5 ms-frame, and a 184-bit control message with a 12-bit CRC for a 20 ms-frame.
A tail bit generator 117 analyzes the output of the CRC generator 115 and adds corresponding tail bits to the output of the CRC generator 115, for terminating an error correction code. Here, the tail bit generator 117 generates 8 tail bits.
An encoder 119 encodes the output of the tail bit generator 117 at a code rate of ⅓. The encoder 119 can be a convolutional encoder or a turbo encoder. An interleaver 121 permutes the bit sequence of encoded symbols received from the encoder 119 in frame units to protect the data from burst errors.
The CRC generator 115, the tail bit generator 117, the encoder 119, and the interleaver 121 form a control message generator 150 for generating a control message and transmitting it on a physical channel. While the control message generator 150 processes a control message for a frame in FIG. 1A, it can be contemplated that the MODEM controller 113 selects a control message generator corresponding to the length of a frame to transmit among as many control message generators as the frame lengths of control messages transmitted on the DCCH. In this case, each control message generator should be provided with a CRC generator, a tail bit generator, an encoder, and an interleaver according to the frame length of a control message processed in the control message generator.
A signal mapper 123 maps 1s and 0s of the interleaved symbols to −1s and 1s, respectively. A gain multiplier 125 performs a DTX mode function by establishing a path for transmitting the DCCH control message or blocking the path depending on which gain control message is received from the MODEM controller 113.
A PCB inserter 127 inserts a PCB into a signal received from the multiplier 125. A serial-to-parallel converter (SPC or S/P) 129 multiplexes control message symbols received from the PCB inserter 127 and distributes the multiplexed symbols to carrier spreaders. Here, three carriers are used by way of example. For the three carriers, six channels are produced from three carrier frequencies and two phases (I and Q channels) of each carrier. The PCB can be used for controlling the reverse link power of a mobile station.
FIG. 1B is a block diagram of a spreader for spreading symbols received from the PCB inserter 127. A forward link transmitter includes as many as spreaders as carriers. One spreader corresponding to one carrier is shown in FIG. 1B. Referring to FIG. 1B, an orthogonal code generator 135 generates a DCCH orthogonal code which can be a Walsh code or a quasi-orthogonal code. Multipliers 131 and 133 multiply I- and Q-channel signals of the forward DCCH control message by the orthogonal code, for orthogonal spreading.
A modulator 137 PN-spreads the orthogonally spread I- and Q-channel signals received from the multipliers 131 and 133 with PN codes PNi and PNq received from a PN sequence generator (not shown). A complex multiplier can be used as the modulator 137.
The above embodiment is a 3× multi-carrier system and can be applied to a transmitter in a 1× or 3× DS (direct sequence) system. Thus, a description of the 1× or 3× DS system will be omitted.
FIG. 2 is a block diagram of a reverse link transmitter which operates in a DTX mode for the general CDMA mobile communication system. As shown in FIG. 2, the reverse link transmitter is similar to the forward link transmitter in structure. Therefore, a description of the same components will be omitted.
An orthogonal spreader 207 generates a Walsh code. A first multiplier 209 multiplies a transmission signal received from a signal mapper 205 by the Walsh code received from the orthogonal spreader 207, for orthogonal spreading. A gain multiplier 221 outputs no data upon receipt of a gain control signal 0 from a MODEM controller 203 if there is no transmission message and outputs data upon receipt of a gain control signal 1 from the MODEM controller 203 if a transmission message exists. A summing device 223 forms a DCCH signal by summing the transmission signal received from the gain multiplier 221 and a pilot/PCB channel signal. A PN spreader 225 complex-PN-spreads the DCCH signal.
FIG. 25A is a block diagram of a base station transmitter in a typical asynchronous IMT-2000 system. A description of the same components as in FIG. 1 will be omitted. In FIG. 25A, a rate converter 2517 changes the rate of the output signal of an encoder 2515 to that of the input signal of an interleaver 2519 by repeating or puncturing the output of the encoder 2515 when the rate at the output of the encoder 2515 is different from that at the input of the interleaver 2519. A multiplexer 2523 time-division-multiplexes a gain-controlled dedicated physical data channel (DPDCH) and a dedicated physical control channel (DPCCH) and feeds the multiplexed signal to a signal mapper 2525.
FIG. 26A is a block diagram of a mobile station transmitter in the typical asynchronous IMT-2000 system. As shown in FIG. 26A, the mobile station transmitter is similar to the base station transmitter in structure. Therefore, a description of the configuration and operation of the mobile station transmitter will be omitted herein.
There will be given a description of the structures and operations of forward and reverse link receivers for performing an outer-loop power control and a closed-loop power control using a reverse pilot channel and a PCB received on a forward DCCH, respectively, with reference to FIGS. 3 and 4.
FIG. 3 is a block diagram of a reverse link receiver in a DTX mode for the general CDMA mobile communication system.
Referring to FIG. 3, a first despreader 301 is a PN despreader for PN-despreading a received signal. A second despreader 303 is a DCCH Walsh despreader for despreading a DCCH signal included in the PN-despread signal received from the first despreader 301 with a Walsh code. A channel estimator 305 detects a fading component using a pilot channel included in the PN-despread signal received from the first despreader 301. A third despreader 307 is a pilot channel Walsh despreader for despreading the pilot channel signal included in the PN-despread signal received from the first despreader 301 with a Walsh code.
A multiplier 314 multiplies the complex conjugate of the fading component received from the channel estimator 305 by the DCCH signal received from the second despreader 303 in symbol units, for error compensation. A PCB extractor 317 extracts a PCB from the error-compensated DCCH signal received from the multiplier 314. A bit energy measurer 309 measures bit energy Eb from the PCB received from the PCB extractor 317 and the fading component received from the channel estimator 305. A noise measurer 311 measures noise energy Nt from the symbol value of the pilot channel received from the third despreader 307 and the fading component from the channel estimator 305. An SNR calculator 313 calculates an SNR from the noise energy Nt and the bit energy Eb. For details of an Eb and Nt measuring method, see “Forward Link Closed Loop Power Control Method for cdma 2000-(Rev. 1)”, Stein Lundby, Contribution to TR45.5.3.1./98.12.08.28.
A decoder 319 decodes the output of the PCB extractor 317 and a CRC error detector 321 performs a CRC error check on the decoded signal received from the decoder 319. The output of the CRC error detector 321 is True (1) or False (0). Since the DCCH channel is transmitted in the DTX mode, the receiver calculates a CRC from a frame if the frame has transmission data to determine whether a frame error has occurred. For details of a method of determining whether a DCCH has frame data or not while in DTX mode, see Korean Application Serial No. 98-04498. A frame detector 322 detects whether a frame exists or not. A data detector 323 receives frame data and a CRC error check result from the CRC error detector 321 and generates an on/off control signal to a MODEM controller 325. The MODEM controller 325 is activated by the on/off control signal to detect a control message from the decoded data received from the decoder 319 and to store the control message in a control message buffer 327.
If the receiver performs closed-loop power control alone, a closed-loop power controller 315 compares the SNR of each PCB received from the SNR calculator 313 with a fixed threshold and controls power according to the comparison result. If the receiver performs closed-loop power control and outer-loop power control together, an outer-loop power controller 329 is further provided to the receiver. The outer-loop power controller 329 determines a threshold and then the closed-loop power controller 315 performs a closed-loop power control using the threshold. The outer-loop power controller 329 is activated upon receipt of a frame existence flag from the data detector 323 and determines the threshold from the CRC check result received from the CRC error detector 321.
Referring to FIG. 6, a closed-loop power control method in the above reverse link receiver will be described.
In step 601, the SNR calculator 313 calculates an SNR from Nt and Eb measured by the noise measurer 311 and the bit energy measurer 309, respectively. Upon receipt of the SNR from the SNR calculator 313, the closed-loop power controller 315 compares the SNR with a fixed threshold in step 603. If the SNR is greater than the threshold, the closed-loop power controller 315 transmits a power-down command (PCB=0) to a mobile station in step 607. If the SNR is equal to or greater than the threshold, the closed-loop power controller 315 transmits a power-up command (PCB=1) to the mobile station in step 605.
FIG. 4 is a block diagram of a forward link receiver in a DTX mode in the general CDMA mobile communication system. The structure and operation of the forward link receiver will be described referring to FIG. 4.
In FIG. 4, a squarer 401 squares an input signal in sub-chip units. An accumulator 403 sums sub-chip energies for one PCG. The sum is estimated as noise energy. A matching filter 405 filters the input signal in sub-chips in chip units. A first despreader 407 PN-despreads the output of the matching filter 405 and outputs the PN-despread signal to a second despreader 409, a channel estimator 411, and a third despreader 413. The third despreader 413 despreads a pilot channel signal included in the PN-despread signal with a Walsh code. An accumulator 415 sums chip energies of the Walsh-spread signal. A squarer 417 squares the sum and outputs the square to an SNR calculator 417. The output of the squarer 417 is estimated as bit energy. The other components are the same as their counterparts shown in FIG. 3 in structure but labeled with different reference numerals. The reverse link receiver also performs a closed-loop power control in the same manner as shown in FIG. 6.
FIG. 5 illustrates DCCH transmission on a forward link and on a reverse link while in DTX mode according to the IS-95C standard. The forward DCCH transmits data discontinuously and PCBs continuously regardless of the presence or absence of data. Also on the reverse link, data is discontinuously transmitted on the DCCH. If no data to be transmitted exists, pilot symbols and PCBs are transmitted on a pilot channel. Hence, the DCCH transmits no PCBs.
In the case of a traffic channel which continuously transmits frames, a receiver can perform outer-loop power control continuously to obtain an intended frame error rate (FER). However, since the DCCH transmits in DTX mode, the outer-loop power control can be used only when transmission frames are present.
FIG. 7 is a flowchart illustrating a general outer-loop power control method. The outer-loop power control method will be described with reference to FIGS. 3 and 7.
As shown in FIG. 7, upon receipt of frame data, the outer-loop power controller 329 determines whether a frame error has been generated based on a CRC error check result received from the CRC error detector 321 in step 701. If a frame error exists, the outer-loop power controller 329 receives a frame existence flag from the data detector 323. If the frame existence flag indicates existence of a frame, the outer-loop power controller 323 increases a threshold in step 703. If the frame existence flag indicates the absence of a frame, the outer-loop power controller 323 decreases the threshold for power control in step 705. Procedures other than the above one can be employed for the outer-loop power control.
When the outer-loop power control method and the closed-loop power control method are used concurrently, a threshold updated for each frame in the outer-loop power control method is used as a reference SNR value in the closed-loop power control method.
FIG. 18A is a block diagram of a mobile station receiver for processing a DPCH received in a DTX mode in an asynchronous IMT-2000 system employed in Japan and Europe. In FIG. 18A, a channel separator 1805 separates a DPCCH from an input DPCH. A channel estimator 1809 obtains information about channel status from the DPCCH received from the channel separator 1805, using pilot symbols. An SNR measurer 1807 calculates pilot energy Eb and noise energy Nt from pilot symbols. A bit energy measurer 1813 receives a DPDCH and the DPCCH, compares their energies, and outputs the comparison result to a data detector. The other components have been described before.
FIG. 18B is a block diagram of a base station receiver for processing a DPCH received while in DTX mode in the asynchronous IMT-2000 system. As shown in FIG. 18B, the base station receiver is similar in structure to the mobile station receiver. Hence, its description is omitted.
As described above, the conventional outer-loop power control method is not applied when no frame exists during a DTX mode on a channel like DCCH since an outer-loop power control is performed based on a determination whether a received frame has an error or not. Therefore, if no frame is transmitted in DTX mode, a threshold set for a previous frame is used.
As a result, when frame transmission resumes and the previous threshold is higher than a threshold which should be set for receiving the current frame without errors, unnecessary transmission power is consumed. On the other hand, if the previous threshold is lower than the desirable threshold, frame errors are increased. The increase of frame errors and transmission power dissipation decrease communication quality and base station capacity.